|Publication number||US6954558 B2|
|Application number||US 10/793,513|
|Publication date||Oct 11, 2005|
|Filing date||Mar 4, 2004|
|Priority date||Jun 24, 2003|
|Also published as||US7127129, US20040264828, US20050244125|
|Publication number||10793513, 793513, US 6954558 B2, US 6954558B2, US-B2-6954558, US6954558 B2, US6954558B2|
|Original Assignee||Intel Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (47), Non-Patent Citations (5), Referenced by (10), Classifications (47), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a continuation-in-part application of and claims priority to application Ser. No. 10/603,410, filed Jun. 24, 2003, now U.S. Pat. No. 6,801,676 entitled “Method And Apparatus For Phase Shifting An Optical Beam In An Optical Device With A Buffer Plug,” and assigned to the Assignee of the present application.
1. Field of the Invention
The present invention relates generally to optics and, more specifically, the present invention relates to modulating optical beams.
2. Background Information
The need for fast and efficient optical-based technologies is increasing as Internet data traffic growth rate is overtaking voice traffic pushing the need for optical communications. Transmission of multiple optical channels over the same fiber in the dense wavelength-division multiplexing (DWDM) systems and Gigabit (GB) Ethernet systems provide a simple way to use the unprecedented capacity (signal bandwidth) offered by fiber optics. Commonly used optical components in the system include wavelength division multiplexed (WDM) transmitters and receivers, optical filter such as diffraction gratings, thin-film filters, fiber Bragg gratings, arrayed-waveguide gratings, optical add/drop multiplexers, lasers and optical switches. Optical switches may be used to modulate optical beams. Two commonly found types of optical switches are mechanical switching devices and electro-optic switching devices.
Mechanical switching devices generally involve physical components that are placed in the optical paths between optical fibers. These components are moved to cause switching action. Micro-electronic mechanical systems (MEMS) have recently been used for miniature mechanical switches. MEMS are popular because they are silicon based and are processed using somewhat conventional silicon processing technologies. However, since MEMS technology generally relies upon the actual mechanical movement of physical parts or components, MEMS are generally limited to slower speed optical applications, such as for example applications having response times on the order of milliseconds.
In electro-optic switching devices, voltages are applied to selected parts of a device to create electric fields within the device. The electric fields change the optical properties of selected materials within the device and the electro-optic effect results in switching action. Electro-optic devices typically utilize electro-optical materials that combine optical transparency with voltage-variable optical behavior. One typical type of single crystal electro-optical material used in electro-optic switching devices is lithium niobate (LiNbO3).
Lithium niobate is a transparent material from ultraviolet to mid-infrared frequency range that exhibits electro-optic properties such as the Pockels effect. The Pockels effect is the optical phenomenon in which the refractive index of a medium, such as lithium niobate, varies with an applied electric field. The varied refractive index of the lithium niobate may be used to provide switching. The applied electrical field is provided to present day electro-optical switches by external control circuitry.
Although the switching speeds of these types of devices are very fast, for example on the order of nanoseconds, one disadvantage with present day electro-optic switching devices is that these devices generally require relatively high voltages in order to switch optical beams. Consequently, the external circuits utilized to control present day electro-optical switches are usually specially fabricated to generate the high voltages and suffer from large amounts of power consumption. In addition, integration of these external high voltage control circuits with present day electro-optical switches is becoming an increasingly challenging task as device dimensions continue to scale down and circuit densities continue to increase.
The present invention is illustrated by way of example and not limitation in the accompanying figures.
Methods and apparatuses for high speed phase shifting an optical beam with an optical device are disclosed. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.
In one embodiment of the present invention, a semiconductor-based optical device is provided in a fully integrated solution on a single integrated circuit chip. One embodiment of the presently described optical device includes a semiconductor-based waveguide having a complementary metal oxide semiconductor (CMOS) capacitor structure, a p-n junction structure or a p-i-n structure, or the like, adapted to modulate a charge concentration along an optical path to phase shift an optical beam in response to a signal. In one embodiment, the charge modulation is to occur in an optical waveguide along an optical path through the optical waveguide. An optical beam is to be directed through the waveguide and through the charge modulated region to phase shift the optical beam. In one embodiment, optical loss due to overlap between the optical mode and a metal contact or a higher doped region is reduced with a buffer of insulating material disposed between the optical path of the optical beam and the metal contact. In one embodiment, a non-uniform doping profile in the optical device enables high speed phase shifting with the higher doped region providing a lower resistor-capacitor (RC) time constant while at the same time reducing optical loss through the optical device. In one embodiment, a buffer plug is also included to help direct the mode of the optical beam away from the metal contact and/or the higher doped region to further reduce optical loss. Embodiments of the disclosed optical devices can be used in a variety of high bandwidth applications including multi-processor, telecommunications, networking as well as other high speed optical applications such as optical delay lines, switches, modulators, add/drops, or the like.
In one embodiment, an optional insulating region 111 is disposed between semiconductor material regions 103 and 105. As illustrated in
In one embodiment, the concentration of charge carriers in charge regions 133 is modulated in response to VSIGNAL in accordance with the teachings of the present invention. In one embodiment, assuming VSIGNAL applies a positive drive voltage VD, the charge density change ΔNe (for electrons) and ΔNh (for holes) is related to the drove voltage VD by
where ε0 and εr are the vacuum permittivity and low-frequency relative permittivity of insulating region 111; e is the electron charge, tox is the thickness of insulating region 111, t is the effective charge layer thickness and VFB is the flat band voltage of the resulting capacitive structure.
In another embodiment, optional insulating region 111 is not included. As such, a p-n junction is formed at the interface between semiconductor material regions 103 and 105. As mentioned in one embodiment above, semiconductor material 103 includes p-type dopants and semiconductor material 105 includes n-type dopants. Depending on how the p-n junction is biased, the concentration of charge carriers in charge regions 133 are modulated in response to VSIGNAL in accordance with the teachings of the present invention. For instance, in one embodiment, the p-n junction may be forward biased or reverse biased as desired in response to VSIGNAL to modulate the concentration of charge carriers in charge regions 133 in accordance with the teachings of the present invention. In another embodiment, it is appreciated that intrinsic material may be included to provide a p-i-n structure or the like in accordance with the teachings of the present invention.
In one embodiment, an optical waveguide 127 is included in optical device 101, through which an optical beam 121 is directed along an optical path. In the embodiment illustrated in
As shown in the embodiment of
In one embodiment, semiconductor material 103 includes a higher doped region 137 at the location at which metal contact 113 is coupled to semiconductor material 103. Similarly, semiconductor material 103 also includes a higher doped region 139 at the location at which metal contact 115 is coupled to semiconductor material 103. In one embodiment, the higher doped regions 137 and 139 are separated by distance WA and are substantially equally spaced from the center of optical waveguide 127, as illustrated in FIG. 1. In one embodiment, semiconductor material 105 includes a higher doped region 141 at the location at which metal contact 117 is coupled to semiconductor material 105. Similarly, semiconductor material 105 also includes a higher doped region 143 at the location at which metal contact 119 is coupled to semiconductor material 105. In one embodiment, the higher doped regions 141 and 143 are separated by distance WD and are substantially equally spaced from the center of optical waveguide 127, as illustrated in FIG. 1.
In an embodiment in which semiconductor material 103 includes p-type dopants and semiconductor material 105 includes n-type dopants, higher doped regions 137 and 139 are heavily doped with p++ type dopants and higher doped regions 141 and 143 are heavily doped with n++ type dopants. In one embodiment, the doping concentration of higher doped regions 137 and 139 is NA(X) and the doping concentration of higher doped regions 141 and 143 is ND(X). In one embodiment, the region between higher doped regions 141 and 143 has a doping concentration of ND, as illustrated in FIG. 1. As also illustrated in the embodiment of
Thus it is appreciated that the semiconductor material of one embodiment of optical device 101 has a non-uniform doping concentration with respect to the y-axis as it may change from 5×1015 cm−3 to ND to NA along the y-axis as illustrated in one embodiment in accordance with the teachings of the present invention. In one embodiment, higher doped regions may be made of semiconductor materials such as silicon, polysilicon, silicon germanium, or any other suitable type of semiconductor material.
Similarly, it is appreciated that the inclusion of higher doped regions 137, 139, 141 and 143 in semiconductor material regions 103 and 105 define a non-uniform doping profile along the x-axis as well in an embodiment of optical device 101 in accordance with the teachings of the present invention. As mentioned above, the portions of semiconductor material 103 and 105 through which the optical path along which optical beam 121 is directed may have lower doping concentrations NA, ND or 5×1015 cm−3 such that optical loss, absorption or attenuation of optical beam 121 is reduced. In addition, the portions of semiconductor material 103 and 105 outside the optical path along which optical beam 121 is directed may have higher doping concentrations, such as NA(X) and ND(X) of higher doped regions 137, 139, 141 and 143. The higher doping concentrations NA(X) and ND(X) of higher doped regions 137, 139, 141 and 143 help improve the electrical coupling of metal contacts 113, 115, 117 and 119 to semiconductor material regions 103 and 105 in accordance with the teachings of the present invention. This improved electrical coupling reduces the contact resistance between metal contacts 113, 115, 117 and 119 and semiconductor material regions 103 and 105, which reduces the RC time constant of optical device 101, which improves the electrical performance of optical device 101 in accordance with the teachings of the present invention. The reduced RC time constant of optical device 101 enables faster switching times and device speed for optical device 101 in accordance with the teachings of the present invention.
To illustrate the non-uniform doping profile of one embodiment optical device 101,
In one embodiment, the doping concentration ND of semiconductor material regions 105 between higher doped regions 141 and 143 with respect to the y-axis of
In one embodiment, the doping concentration NA of semiconductor material regions 103 below higher doped regions 137 and 139 with respect to the y-axis is also relatively low doping concentration. In one embodiment, NA is approximately equal to ND. In one embodiment, NA is approximately equal to and slightly less than ND. In one embodiment, the wider upper part with respect to the y-axis of semiconductor region 103, which includes higher doped regions 137 and 139, has a relatively shallow doping depth having a thickness of 0.15 μm in one embodiment. As shown in
It is noted that embodiment shown in
Referring back to the embodiment illustrated in
In one embodiment, a buffer of insulating material 123 and a buffer of insulating material 125 are also included in an optical device 101 in accordance with the teachings of the present invention. As shown in
In one embodiment, a buffer plug 135 of insulating material may also be disposed in optical waveguide 127. In another embodiment, buffer plug 135 of insulating material is not included in optical waveguide 127. As shown in the example embodiment of
In operation, optical beam 121 is directed through optical waveguide 127 along an optical path through charge regions 133. VSIGNAL is applied to optical waveguide 127 to modulate the free charge carrier concentration in charge regions 133 in semiconductor material 103 and 105. In the embodiment including insulating layer 111, the charge regions are proximate to insulating 111. In the embodiment without insulating layer 111, the charge regions 133 may be proximate to the interface between semiconductor material regions 103 and 105 or throughout the optical waveguide, depending on how the p-n junction is biased. The applied voltage from VSIGNAL changes the free charge carrier density in charge regions 133, which results in a change in the refractive index of the semiconductor material in optical waveguide 127.
In one embodiment, the free charge carriers in charge regions 133 may include for example electrons, holes or a combination thereof. In one embodiment, the free charge carriers may attenuate optical beam 121 when passing through. In particular, the free charge carriers in charge regions 133 may attenuate optical beam 121 by converting some of the energy of optical beam 121 into free charge carrier energy. Accordingly, the absence or presence of free charge carriers in charge regions 133 in response to in response to VSIGNAL will modulate optical beam 121 in accordance with the teachings of the present invention.
In one embodiment, the phase of optical beam 121 that passes through charge regions 133 is modulated in response to VSIGNAL. In one embodiment, the phase of optical beam 121 passing through free charge carriers in charge regions 133, or the absence of free charge carriers, in optical waveguide 127 is modulated due to the plasma optical effect. The plasma optical effect arises due to an interaction between the optical electric field vector and free charge carriers that may be present along the optical path of the optical beam 121 in optical waveguide 127. The electric field of the optical beam 121 polarizes the free charge carriers and this effectively perturbs the local dielectric constant of the medium. This in turn leads to a perturbation of the propagation velocity of the optical wave and hence the index of refraction for the light, since the index of refraction is simply the ratio of the speed of the light in vacuum to that in the medium. Therefore, the index of refraction in optical waveguide 127 of optical device 101 is modulated in response to the modulation of free charge carriers charge regions 133. The modulated index of refraction in the waveguide of optical device 101 correspondingly modulates the phase of optical beam 121 propagating through optical waveguide 127 of optical device 101. In addition, the free charge carriers in charge regions 133 are accelerated by the field and lead to absorption of the optical field as optical energy is used up. Generally the refractive index perturbation is a complex number with the real part being that part which causes the velocity change and the imaginary part being related to the free charge carrier absorption. The amount of phase shift φ is given by
φ=(2π/λ)ΔnL (Equation 2)
with the optical wavelength λ, the refractive index change Δn and the interaction length L. In the case of the plasma optical effect in silicon, the refractive index change Δn due to the electron (ΔNe) and hole (ΔNh) concentration change is given by:
where no is the refractive index of intrinsic silicon, e is the electronic charge, c is the speed of light, ε0 is the permittivity of free space, m− e* and mh* are the electron and hole effective masses, respectively, be and bh are fitting parameters. The optical absorption coefficient change Δα due to free charge carriers in silicon are given by
where λ is the wavelength of light in free space, c is the velocity of light in a vacuum, no is the refractive index of intrinsic silicon, m*e is the effective mass of electrons, m*h is the effective mass of holes, μe is the electron mobility and μh is the hole mobility.
For instance, in one embodiment, the width WR of the rib region 129 of optical waveguide 127 is approximately 2.5 μm, the height HR of the rib region 129 of optical waveguide 127 is approximately 0.9 μm and the height HS of the slab region 131 of optical waveguide 127 is approximately 1.5 μm. In one embodiment, the thickness of buffer regions 123 and 125 is approximately 0.5 to 0.8 μm and the thickness of the semiconductor material region 103 between contacts 113 and 115 and buffer regions 123 and 125 is approximately 0.2 to 0.3 μm.
As illustrated in the example embodiment of
In one embodiment, it is noted that by reducing the distance D between contacts 113 and 115 and charge regions 133, the speed of optical device 101 may be further increased due to the reduced RC time constant of the device. Furthermore, as stated previously, with the inclusion of higher doped regions 137 and 139, the electrical coupling between contacts 113 and 115 and optical waveguide 127 is further improved, which further reduces RC time constant of the optical device 101 in accordance with the teachings of the present invention.
Therefore, in one embodiment, metal contacts 113 and 115 may be located very close to the center of optical waveguide 127 in accordance with the teachings of the present invention with substantially little or no optical loss due to contacts 113 and 115 while the operating speed is still high. Indeed, it is appreciated that without buffers 123 and 125, a relatively high amount of optical loss may result due to an overlap between the optical mode of optical beam 121 and contacts 113 and/or 115.
To illustrate a relationship between phase modulation efficiency of various embodiments of optical device 101 with different dimensions,
As can be appreciated from
It is appreciated of course that the precise device speeds, modulation bandwidths, optical losses, doping concentrations, materials etc. have been provided herewith for explanation purposes and that other suitable values or materials may be chosen in other embodiments or applications in accordance with the teachings of the present invention. As mentioned, it is appreciated that by scaling down dimensions such as WA, WD, WR, HR, HS and the thickness of insulating region 111 as well as adjusting the doping concentrations up or down of NA, ND, NA(X) and ND(X) is discussed herein for suitable applications, very high speed bandwidth operation with acceptable electrical characteristics and reduced optical loss are now possible with an optical device 101 in accordance with the teachings of the present invention.
Referring now back to the embodiment of
For instance, in one embodiment of the present invention, a semiconductor-based optical amplitude modulator is provided in a fully integrated solution on a single integrated circuit chip. In particular,
In operation, an optical beam 721 is directed into an input of MZI configuration 705. Optical beam 721 is split such that a first portion of the optical beam 721 is directed through one of the arms of the MZI configuration 705 and a second portion of optical beam 721 is directed through the other one of the arms of the MZI configuration 705. As shown in the depicted embodiment, one of the arms of the MZI configuration 705 includes optical phase shifter 703, which adjusts a relative phase difference between the first and second portions of optical beam 721 in response to signal VSIGNAL. In one embodiment, the first and second portions of optical beam 721 are then merged in the semiconductor substrate such that optical beam 721 is modulated at the output of MZI configuration 705 as a result of constructive or destructive interference. In one embodiment, as shown, one of the arms of the MZI configuration 705 includes an optical phase shifter 703. In another embodiment, both of the arms of the MZI configuration 705 may include an optical phase shifter 703 in accordance with the teachings of the present invention. In various embodiments according to the teachings of the present invention, it is appreciated that optical phase shifter 703 can be designed with scaled down waveguide dimensions and non-uniform doping concentrations and profiles operate at high speeds such as for example 10 GHz and beyond without excessive optical loss is discussed above.
In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
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|U.S. Classification||385/3, 359/248, 248/637, 385/130, 385/4, 359/279, 385/2, 378/65, 385/1, 378/144, 248/121, 378/208, 385/129, 248/122.1, 378/68, 248/218.4, 248/123.11, 378/196, 248/575, 385/131, 378/195, 250/491.1, 248/219.2, 378/143, 248/672, 378/20, 248/674, 378/210, 378/4, 250/492.3, 378/197, 378/15, 248/127, 385/132, 248/612, 378/198, 248/676, 250/396.00R, 248/638|
|International Classification||G02B6/26, G02B6/10, G02F1/025, G02F1/313|
|Cooperative Classification||G02F1/025, G02F1/3133|
|European Classification||G02F1/025, G02F1/313C2|
|Mar 4, 2004||AS||Assignment|
Owner name: INTEL CORPORATION, CALIFORNIA
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Effective date: 20040303
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